1. Field of the Invention:
The present invention relates to thermosetting, solvent-based polyurethane polyols. More specifically, the invention relates to an improved thermosetting, solvent-based, high solids coating comprising polyester-urethane polymers that are useful in coating both elastomeric and metal substrates, and an improved method of preparing same.
2. Brief Description of the Prior Art:
There are known in the art coating compositions that can be applied to elastomeric substrates. These coating compositions are characterized by excellent flexibility, particularly at low temperatures. Illustrative of these coating compositions are those based on polyester-urethane polyols of relatively high molecular weight which are cured with aminoplasts. Another illustration of these coating compositions are those based on polymeric polyols such as soft polyurethane polyols cured with a stoichiometric excess of aminoplast curing agents. Yet another illustration of these coating compositions are those comprising thermosetting resins, for example, hard polyester polyols, soft polyurethane polyols and aminoplast curing agents. It has been found that while these elastomeric coatings have a reasonable degree of hardness, they are not sufficiently hard to be used effectively on hard metallic substrates.
Coating compositions that can be applied to rigid substrates such as hard metallic substrates are known in the art. These coating compositions are characterized by excellent hardness. An illustration thereof are coating compositions based on polyester polyols and acrylic polymers cured with aminoplasts or isocyanates. These metallic coatings, generally, are not flexible enough to be used effectively on elastomeric substrates.
In this state of the art, an article, with rigid and elastomeric parts, requires two types of coatings for the final topcoat: one suitable for elastomeric parts and the other suitable for rigid parts. For pigmented topcoats, this requirement presents problems of color-matching of the different parts. Those skilled in the art would appreciate how difficult it is to prepare durable coatings having, conjointly, the properties of flexibility and hardness. It would be all the more difficult to prepare high solids compositions having the conjoint properties. In commonly assigned co-pending U.S. applications Ser. Nos. 538,630 (now U.S. Pat. No. 4,485,228, granted Nov. 27, 1984) and 538,631, there are disclosed high solids coating compositions, the cured coating of which has, conjointly, excellent properties of flexibility, durability and hardness which make it useful for coating on both elastomeric and hard metal parts. The disclosed coating compositions comprise polyester-urethane polyols and methods of preparing same. Typically, the polyester-urethane polyols are prepared by reacting a polyisocyanate with a polyester polyol formed from reacting a polycarboxylic acid and a polyol, or functional equivalents thereof.
The present invention provides an alternate method of preparing polyurethane polyols useful for coatings that have the conjoint properties of hardness and flexibility. Notably, the preparation of the polyurethane polyol useful herein does not entail the use of polyisocyanates.